Vane-type angle of attack sensors typically are mounted to the fuselage or other exterior structure of an aircraft so that the sensor vane will be angularly deflected by airflow passing along the surface of the aircraft at the location of the sensor. In various flight control systems, the signal supplied by such a vanetype sensor (referred to as an alpha vane signal) is converted to a signal representative of the aircraft body angle of attack, which is used to establish gain scheduling and/or open loop compensation.
In certain situations, the alpha vane signals supplied by currently available sensors and the methods and arrangements for converting the alpha vane signals to aircraft body angle of attack signals are at least somewhat undesirable. One such situation arises with respect to automatic landing systems that guide an aircraft along glideslope and localizer beams so that the aircraft approaches and lands at a destination runway without manual control by the pilot.
In automatic landing systems, the problem presented by prior art vane-type angle of attack sensors relates to ground effects that occur during the landing procedure when the aircraft altitude is less than approximately one wingspan. Specifically, as the aircraft approaches the ground, the local flow in front of the wing changes its upward motion due to ground compression. These ground effects result in airflow disturbances that cause prior art vane-type angle of attack sensors to supply erroneous signals. Since the body angle of attack signals that are obtained from the alpha vane signal are used in an automatic landing system for gain scheduling and open loop compensation in various control laws that control the aircraft throughout the landing procedure, ground effect induced signal errors can affect important automatic landing characteristics such as vertical path control of the aircraft and touchdown dispersion. For example, in some instances, ground effect induced errors in the alpha vane signal (and, hence the aircraft body angle of attack signal) may be incorrectly interpreted by the automatic landing system as a vertical wind gust. In these instances, the automatic landing system may initiate unnecessary control action which, unless counteracted by additional system control action, will result in less than optimal landing performance.